1. Field of the Invention
The invention relates to a compressed gas circuit breaker comprising two fixed hollow contacts which are bridged by a tubular bridging contact when the breaker is in the closed position. More particularly, it relates to a circuit breaker of the aforesaid type in which a blasting means provides a compressed quenching gas for blasting the arc drawn between the bridging contact and the fixed contacts during opening of the breaker, and in which the blasting means includes a piston and a blasting cylinder surrounding the piston and having an end portion which is moved from one fixed contact to the other via the gap therebetween during the opening of the breaker.
2. Description of the Prior Art
Such a compressed gas circuit breaker is disclosed in the U.S. Pat. No. 3,789,175. In the closed position of the breaker, the blasting cylinder surrounds one fixed contact, forming a seal therewith. As the bridging contact and, hence, the blasting cylinder are moved, the sulfurhexafluoride (SF.sub.6), gas provided as the quenching medium is compressed by the motion of the blasting cylinder in relation to the piston, which is maintained stationary. When the bridging contact no longer bridges the fixed contacts, i.e., when it ceases to contact the aforesaid one fixed contact, an arc is formed under load, the latter arc, in turn, being blasted and cooled by the compressed quenching gas. In the process, the quenching gas is driven into the gap between the two fixed contacts and therefrom into the interior of the contacts for the discharging of same.
In designing the aforesaid compressed gas circuit breaker, it is desirable to keep the gap between the fixed contacts as small as possible. However, when switching capacitive currents which result in comparatively high recurring voltages, the use of a small gap causes uneven surface charges to exist on the insulating blasting cylinder. At high field strengths, this uneven surface charge can lead to a breakdown and thus to a recurrence of the arc.
It is an object of the present invention to provide a compressed gas circuit breaker of the type described above which has both an increased protection against arc recurrence and a small gap between its fixed contacts.